txRDQ provides a ResizableDispatchQueue class that gives you a priority queue
to operate on a priority queue of jobs. Jobs can be canceled or reprioritized.
The size of the underway job pool can be changed dynamically.  Dispatching can
be paused and resumed. The queue can be stopped gracefully, and will return
jobs that had not yet been dispatched.  You can obtain the contents of the
queue and the dispatched pool while running.
